Carbon-carbon composites are useful materials for high temperature applications. Characteristics such as high heat of ablation, thermal shock resistance, strength improvement at elevated temperatures, and chemical inertness result in a material that is capable of high performance in extreme thermal environments. Carbon-carbon composites consist of a fibrous carbon substrate in a carbonaceous matrix wherein each constituent may range from carbon to graphite. The temperature capabilities of these composites extend to over 6000.degree. F. and the strength of the composites is about twenty times that of graphite, yet they are lighter and have a density of less than 1.8 grams per cubic centimeter. Since carbon-carbon composites oxidize in air at temperatures above 400.degree. C., such composites require a coating to protect them from oxidation.
A typical carbon-carbon composite is formed from graphite cloth, impregnated with a carbonaceous polymer or resin, which is laid in a form or mold and cured. After trimming, the material is pyrolyzed to convert the polymer or resin to graphite. The soft composite is reimpregnated and repyrolyzed as many times as is necessary to form the composite of correct strength and density. The composite can be used as is or can be coated to protect it from severe conditions of use.
One coating technique disclosed in U.S. Pat. No. 4,321,298 involves coating a fibrous carbon material containing boron with a flexible thermosetting resin which contains a refractory metal such as tungsten or molybdenum. The advantage to this technique is that a metal boride is formed at high temperatures which is more stable than boron carbide. However, no mention is made to applying this technique to carbon-carbon composites without boron. In fact the invention is aimed at preventing the detrimental interaction of boron and the carbon fibers.
Additionally, U.S. Pat. No. 4,618,591 teaches a method of preparing a silicon carbide-carbon composite. The process involves using a polycarbosilane to impregnate a base material. Patentee shows no appreciation regarding the high temperature resistance to oxidation of these composites. Since the composite contains free carbon it would not be expected to be very stable at high temperatures.
In contrast to this prior art, the instant invention discloses a method of forming a high temperature oxidation resistant coating comprising a black glass on a carbon-carbon composite. This extends the usable temperature range for these carbon-carbon composites.